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Full text of "Treatise On Analysis Vol-Ii"

400       XV    NORMED ALGEBRAS AND SPECTRAL THEORY

For every eigenvalue a ofN,f(a) is an eigenvalue off(N), and the eigenspace
E(a; N) is contained in E(/(a);/(JV)).

Iff is continuous, then Sp(/(JV)) =/(Sp(J/V)).

(ii)    More precisely, with the notation of (1 5.1 1 .3) , iffe *c(sP(^ii))
f/ze spectrum off(Nn) consists of the complex numbers ft such that

essinf|/?-/(C)l =

C 6 Sp(JVn)

(relative to the measure un).

(iii)    Iffe <%c(Sp(N)) fl/zrf z/# w a continuous mapping off(Sp(N)) into C,

(iv) 7f a sequence (fk) of functions belonging to ^c(Sp(JV)) ^ uniformly
bounded and converges simply tof, then for every x e E the sequence (fk(N)  x)
converges in E tof(N)  x.

We shall start by proving (ii). We have seen that/^J may be identified
with multiplication by the class of the function /in Lc(Sp(JVM), un). Hence
p $ SpC/X-AQ) if and only if there exists a real number a > 0 such that
N2(<j3-/>)^tf-N2(w) for all functions u e ^(Sp(^), /O (5.5.1). If
ess inf ]/?  /(Ol > ^> then we may take a to be equal to this number, by

C. e Sp(tfn)

virtue of (13.12.2), and therefore p $ Sp(f(Nn)). Conversely, if

essinf|/?~/(C)| = 0,

C e Sp(JVnX

then for each e > 0 the set M of complex numbers C e Sp(7Vn) such that
I0-/(OI ^'e is not ^-negligible, and we have N2(0? -f)(pM) ^ eN2(<pM),
hence / e Sp(/(JVJ).                               _

It follows from (ii) that Sp(f(Nn)) c/(Sp(A^n)). If, moreover, / is con-
tinuous, then f(Sp(Nn)) is compact (3.17.9), and for each j5 =/(a), where
aeSp(Afn), every compact neighborhood of a has /vmeasure >0 (15.11.4),
so that/(Sp(J\g) = $p(f(NJ). To show that Sp(/(JV))c:/(Sp(N)) in general,
and that Sp(/(N)) =/(Sp(N)) when / is continuous, we have only to use
(15.11.5) and the fact that /(Sp(A/)) is compact if /is continuous. Finally,
the assertions about eigenvalues are evident, again by reducing to the case of
simple operators and using (15.11.6).

To prove (iii), note first that g /is a universally measurable mapping of
Sp(JV) into C (13.9.6). The relation

follows from (15.11.1) in the case where #(Q = p%q (/?, q being integers ^
Now, for each e>0, there exists a polynomial h in  and  such that
l<7(0 - h(Q\  e for all C e/(Sp(]V)) (7.3.2). Since |^(/(0) - A(/(0)| ^ e fore a, then